1. Field of the Invention
The present invention relates to a charged particle beam processing method for processing with high accuracy the surface of an object, such as an optical element or a mold for forming the optical element, using charged particle beams.
2. Description of the Related Art
To form an optical validity region with desired optical characteristics on the surface of an optical element, the shape or roughness of the optical validity region should be processed with more than a certain degree of accuracy. The shape, in this invention, means a residual shape in a space wavelength band of equal to or more than 1 mm, and the roughness means a residual shape in a space wavelength band of equal to or less than 1 μm. Since the charged particle beam processing can be performed while being electrically controlled, it has high controllability. In addition, since the amount of processing is determined by the number of particles reaching the surface of the object, it is possible to process an object with high accuracy. Therefore, the charged particle beam processing method is used to process the surface of an object, such as an optical element.
In a shape correcting process using the charged particle beam processing method, first, the shape of the surface of an object is measured, and a residual shape is calculated from the difference between the measured shape and the design shape. The residual shape is removed as a target processed shape to process the surface of the object into the design shape. The number of particles of the charged particle beam that reach the surface of the object is controlled according to positions such that the target processed shape is processed. The number of particles of the charged particle beam that reach a unit area of the surface of the object (the number of particles that reach a unit area) can be represented by the product of the radiation time and the amount of current of the charged particle beam, on the condition that the valence of the charged particle is known. Therefore, a predetermined charged particle beam with a diameter that is smaller than that of the surface of an object and with a constant current is radiated to the surface of the object while the beam being moved relative to the surface of the object, and the radiation time is controlled by the relative movement in which a residence time distribution corresponding to a target processed shape. In this way, a shape correcting process is performed (see Japanese Patent Application Laid-Open No. 2007-098438).
When the roughness after the shape correcting process is performed is larger than a desired roughness, a smoothing process is necessary to be additionally performed. Examples of a smoothing processing method using a charged particle beam include: a processing method using a low-energy charged particle; a processing method using a charged particle beam that is obliquely incident; and a processing method after a film is formed. In each processing method, the smoothing process, in which the roughness is reduced without changing the shape, is controlled such that the number of particles of the charged particle beam that reach a unit area of the surface of the object is constant, and the entire processing region is uniformly processed.
In addition, when charged particle beam processing is performed plural times, plural charged particle beams are radiated to the surface of an object at the same time and processing is performed in parallel, thereby reducing the processing time (see Japanese Patent Application Laid-Open No. H04-373125).
In the method disclosed in Japanese Patent Application Laid-Open No. H04-373125, a portion of the object requiring finish processing is limited. Therefore, even when roughness processing and finish processing are independently performed, it is possible to reduce the processing time.
However, in the method disclosed in Japanese Patent Application Laid-Open No. 2007-098438 in which the smoothing process is performed after the shape correcting process, it is most efficient to radiate each of the charged particle beams to an object substantially at the same time by the same movement relative to the object. However, the method in which plural charged particle beams are simultaneously radiated to the surface of the object with the same relative movement and the shape correcting process and the smoothing process are performed in parallel to reduce the processing time has the following problems.
In the shape correcting process and the smoothing process performed on the surface of the object, different numbers of particles are controlled to reach a unit area. That is, in the structure in which the number of particles that reach a unit area is controlled by the relative movement between the charged particle beam and the surface of the object, the shape correcting process and the smoothing process performed on the surface of the object need to have different relative movements between the charged particle beam and the surface of the object. Therefore, it is difficult to simultaneously radiate plural charged particle beams to the surface of the object and perform the shape correcting process and the smoothing process on the surface of the object in parallel while moving the charged particle beams relative to the surface of the object.